1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus that generates a projection image by rendering volume data acquired by transmission and reception of ultrasonic waves and displays the projection image.
2. Description of the Related Art
With the rapid enhancement of computation by computers in recent years, it has become possible to generate a three-dimensional moving image by rendering 4D data acquired by continuously scanning the inside of a subject, and to make a diagnosis and provide treatment based on the three-dimensional moving image. For example, as shown in Japanese Unexamined Patent Application Publication No. 2004-275223, an ultrasound diagnosis apparatus can also acquire 4D data with a two-dimensional array ultrasound probe capable of three-dimensionally transmitting and receiving an ultrasonic beam to and from a space inside a subject and generate a three-dimensional moving image by rendering the 4D data.
Rendering is creation of a projection moving image representing a three-dimensional figure by projecting a region to be rendered in a viewing direction. A rendering method is, for example, the surface display method and the volume rendering method. In the volume rendering method, voxel data of a region viewed from a viewpoint is sampled, transmission of light in accordance with opacity and reflection to the viewpoint are calculated, and a projection moving image is generated while shading is applied.
In this rendering, it is necessary to properly specify a viewpoint and a region to be rendered. Moreover, an ultrasound diagnosis apparatus configured so that a scanning region to and from which ultrasonic waves are transmitted and received is variable also needs to properly specify the scanning region. With the rapid enhancement of computation by computers in recent years, it has become easier to change the aspects of rendering, e.g., change a viewpoint, change a region to be rendered, and change a region to be scanned. When an operator inputs change of the viewpoint and the region, the ultrasound diagnosis apparatus immediately executes re-rendering and displays an image.
However, since the operation of changing the aspects of rendering has become possible, such a problem has arisen that it is impossible to display a projection moving image taken from a desired reference cross section unless the operator properly designates the viewpoint and the region to be rendered, or properly designates a region to be scanned, the viewpoint and a region to be rendered. For example, it is assumed that one region across a reference cross section is scanned and the one region is rendered in a state that a viewpoint is set so that the reference cross section is in front and the one region is on the back. If the operator changes the position of the viewpoint from the above state to the reverse side beyond the reference cross section, the rendering is executed in an aspect that a plane opposite to the reference cross section is the surface, with the result that it is difficult to observe the reference cross section from the projection image.
In other words, when performing a viewpoint changing operation, in response to the operation, the operator must perform a changing operation to scan the other region across the reference cross section, and further perform a changing operation to render the other region. If not, a projection image with a reference cross section viewed from the changed viewpoint as the surface cannot be generated.